3.2 Evolution of Elastic Modulus and Poisson’s ratio of
thermal treated granite with Cyclic Number
Figure 2 shows the variation of elastic modulus (E ) with cycle
number, Young’s modulus represents the slop of stress-strain curve in
linear segment under loading stage (30%-70% of the peak strength in
each cyclic loading). It is clear that elastic modulus first increases
and enter a stable phase, then decreases before becoming constant again
with cycle number. In the early loading stage, pre-existing fissures
closed and result in the increase of elastic modulus. After that, the
loading enter to the elastic stage, few crack initiate in this loading
stage, and the elastic modulus remains near constant. With the increase
of cycle number, more micro-cracks were induced by compression, and
macro-crack formed gradually, which result in the decrease of elastic
modulus. However, after the loading enter to the residual strength
stage, and the elastic modulus reach to a plateau again.
Fissures pre-closed when applied confining pressure partially, and make
the increasing stage of elastic modulus shorter than that under uniaxial
compression. It is also clear that elastic modulus increases with the
increase of confining pressure at the same cycle number, this phenomenon
is more obvious in the granite specimens treated by 600°C. After high
temperature treatment, more cracks are induced, the potential for cracks
to close is great, and make the elastic modulus more sensitive to
confining pressure. Confining pressure also restrain crack propagation
under loading process, therefore the elastic deformation stage is more
obvious with the increase of confining pressure, and the decreasing rate
of elastic modulus with cycle number decreases in the macro-crack formed
stage. Confining pressure also increases the contact area of the
macro-crack, and result in the increase of elastic modulus in the
residual strength stage.
Figure 3 depicts Poisson’s ratio of granite after thermal treatment
under different confining pressure. Poisson’s ratio represents the ratio
between radial and axial strain in the elastic stage, it may be larger
than 0.5 after the macro-cracks formed (not in the elastic deformation
stage). Poisson’s ratio remains near constant in the initial stage, then
increases rapidly, and decreases slightly before entering a stable phase
again with cycle number on the whole. The slight decreases of Poisson’s
ratio are also obtained by Xiong et al (2019). It indicates that there
is few crack induced by loading in the initial stage, and the radial
strain is not evident. After that, more vertical micro-cracks are
formed, and it re-open in the elastic stage, which result in the
expansion in radial direction. However, slippage occurred along
macro-crack, and lead to the re-open for vertical micro-cracks
decreases, when the loading to the residual strength stage. Therefore,
Poisson’s ratio slightly decreases before entering a stable phase again.
Poisson’s ratio of specimen under uniaxial compression is lesser than
that under triaxial compression at initial loading stage when T =
25°C and 300°C, as shown in Figs. 3a-b. It can be explained as that
pre-existing fissure closed when applied confining pressure, and result
in the decrease of potential for grain to adjustment under loading
process, therefore the specimen is easier to expansion in radial
direction. The splitting tensile crack is easier to initiate in specimen
under uniaxial compression, and it re-open under loading process, which
make the increasing rate of Poisson’s ratio higher than that under
triaxial compression. Confining pressure can restrain micro-cracks
re-open, and Poisson’s ratio decreases with increasing confining
pressure on the whole.
When T = 600°C, more thermal cracks are induced in specimen,
which lead to the increases of Poisson’s ratio with cycle number once
applied compression as shown in Figs. 3c. Lacking confining pressure,
Poisson’s ratio of specimen under uniaxial compression is larger than
that under triaxial compression. After macro-cracks formed, the
decreasing rate of Poisson’s ratio with cycle number is larger than that
when = 25°C and 300°C, it indicates that there are more vertical cracks
in specimen when T = 600°C.